1. Field of the invention
The present disclosure relates to multipolar power cables, particularly for the transport or distribution of low, medium, or high voltage electrical power, having impact resistant properties, and to a process for the production thereof.
More particularly, the present disclosure relates to impact resistant multipolar power cables comprising a plurality of cores stranded to form an assembled element with interstitial zones between the cores; an expanded polymeric filler that fills the interstitial zones; and an impact resistant, expanded polymeric layer radially external to and in contact with the expanded polymeric filler.
2. Background
Within the scope of the present disclosure, “low-voltage” generally means a voltage less than about 1 kV, “medium-voltage” means a voltage between 1 kV and 35 kV, “high-voltage” means a voltage greater than 35 kV.
Electrical cables generally comprise one or more conductors, individually coated with insulating and, optionally, semiconductive polymeric materials, and one or more protective coating layers, which can also be made of polymeric materials.
Accidental impacts on a cable, which may occur, for example, during their transportation, laying and operation, may cause structural damage to the cable, including deformation or detachment of insulating and/or semiconductive layers, and the like. This damage may cause variations in the electrical gradient of the insulating coating, with a consequent decrease in the insulating capacity of this coating.
Commercially available cables, for example those for low- or medium- or high-voltage power transmission or distribution, provide metal armour or shield capable of withstanding such impacts. This armour/shield may be in the form of tapes or wires (generally made of steel), or alternatively in the form of a metal sheath (generally made of lead or aluminium). This armour with or without an adhesive coating is, in turn, often clad with an outer polymer sheath. An example of such a cable structure is described in U.S. Pat. No. 5,153,381.
Applicants have observed that the presence of the above mentioned metal armour or shield, however, has a certain number of drawbacks. For example, the application of the said armour/shield includes one or more additional phases in the processing of the cable. Moreover, the presence of the metal armour increases the weight of the cable considerably. In addition, the metal armour/shield may pose environmental problems since, if it needs to be replaced, a cable constructed in this way is not easy to dispose.
To make more light weight and flexible cables, expanded polymeric materials have replaced metal armour/shields while still maintaining impact and, at least to a certain degree, flame and chemical resistance. For example, a solid interstitial filler overlaid with an expanded polymeric layer may provide excellent impact resistance, such as described in U.S. Pat. No. 7,601,915. However, flexibility and weight of the cable is sacrificed.
Alternatively, an expanded polymeric material may fill the interstitial volume between and overlay the core elements present in the inner structure of the cable. U.S. Pat. No. 6,501,027 describes a power cable comprising an expanded polymeric filler in the interstitial volume between the cores with an outer sheath coating. The expanded polymeric filler is obtained from a polymeric material which has, before expansion, a flexural modulus higher than 200 MPa. The polymer is usually expanded during the extrusion phase; this expansion may either take place chemically, by means of a compound capable of generating a gas, or may take place physically, by means of injection of gas at high pressure directly into the extrusion cylinder. The outer sheath, which is a non-expanded polymeric layer, is subsequently extruded over the expanded polymeric filler.
U.S. Pat. No. 7,132,604 describes a cable with a reduced weight and a reduced amount of extruded material for the outer sheath and comprising a polymeric material filler and an expanded sheathing material surrounding the filler. The expanded sheathing material can be any material that has a tensile strength between 10.0 MPa and 50.0 MPa. The expansion rate of the sheathing material can be from 5% to 50%. The material of filler can be a material on the basis of polyvinylchloride, rubber, EPDM (Ethylene Propylene Terpolymer) or POE (Poly Olefin Elastomer). The filler can be made of expanded material. The expansion rate of the filler can be from 10% to 80%.
U.S. Pat. No. 7,465,880 teaches that applying an expandable polymeric material to the interstitial zones of a multipolar cable is a complex operation which requires special care. An incorrect application of such material inside of the interstitial zones of the assembled element will result in the occurrence of unacceptable structural irregularities of the cable. The polymeric material, which is applied to the interstitial zones by extrusion, expands more in the portion of the interstitial zone that has the most space available to expand and the resulting transverse cross section of the semi-finished cable has an external perimetral profile which is substantially trilobate.
To overcome the non-uniform and non-circular expansion of polymeric filler, U.S. Pat. No. 7,465,880 teaches to deposit the filler made of expandable polymeric material by co-extrusion with a containment layer of non-expanded polymeric material. An optimum mechanical strength against accidental impacts is conferred to the cable of U.S. Pat. No. 7,465,880 by arranging a layer of expanded polymeric material in a position radially external to the containment layer.
U.S. Patent Application Publication No. 2010/0252299 describes a cable comprising a conductor core, a polymeric material filler and an armour layer. A foaming agent may be configured to create voids in the filler. After being extruded onto the conductor core, the filler may have a squeezing force applied to its exterior by armour. The armour is configured to squeeze the voids in the filler.